The queasy apprehension of pure evil is what Errol Morris's new documentary appears to offer; it is a series of interviews with those people responsible - some of them, anyway - for the Abu Ghraib prison scandal of 2003. Digital photos showed US soldiers clowning around for the camera while they brutalised and dehumanised Iraqi prisoners. And Morris's movie shows that it was not just a matter of humiliating these people and robbing them of their dignity. One picture shows a soldier grinning next to the ice-packed corpse of a man who died while in the "stress" position: in other words, a man who had been tortured to death. Eventually it was the responsibility of the US military police to decide which of the pictured events constituted torture, and which were "standard operating procedure". No one higher than the rank of staff sergeant was ever brought to book, despite the fact that a vaguely defined policy of "fucking with" the prisoners was sanctioned from the very top.

You may think, as I did, that you have already seen these photos in the press, and got over the revulsion. But only a few were widely published and these often in restricted or blurred form. Some have been shown in Alex Gibney's documentary Taxi to the Dark Side, but seeing so many of them now, in full, at length, and at cinema-screen size, is a bone-chilling experience. For me, it is as if someone had discovered and shown the semi-mythical cine-film that Josef Goebbels was rumoured to have taken of the Stauffenberg plotters being hanged, or as if the audio tapes of Ian Brady or Myra Hindley torturing children were now being made available on iTunes. Perhaps the worst picture of all is the horrifying image of a man in a pointed hood, made to stand, arms out, on a narrow block, in a mock electrocution ceremony. It is simply satanic: a hard-core porn image of unalloyed horror.

If anything, the interviews are even more disturbing. One respondent, the notorious Lynndie England, appears dead behind the eyes, and entirely unrepentant. It is easy to imagine how such a person, scared and bored in any case by life in the unspeakable prison, could have been manipulated by a man higher up the chain of command with whom she was avowedly in a sexual relationship. And it is all too clear how a situation like Lord of the Flies crossed with the Stanford prison experiment could have arisen, especially as the soldiers had convinced themselves that this behaviour was an alternative, if boisterous alternative, to torture: a kind of frat-house initiation. How about the other interviewees? They seem rational, even likable. At first, they seem like witnesses for the prosecution, rather than defendants and it isn't at all clear who are the good guys and who are the bad guys. Morris quotes a letter home from one who claims that taking these photos was the only way to prove the abuses were happening, and encouraging the clowning around was the only way to ensure her camera was not confiscated. Is that true? Maybe. But it would have been interesting to hear when exactly this letter was written.

And what happened to the victims in the end, these unnamed people for whom none of Morris's interviewees expresses a moment of real sympathy or even curiosity? The pictures show empty cells, blood-washed floors. The film's central moment comes when one soldier spins a dubious yarn for Morris. He claims that one of the local Iraqi guards smuggled a handgun into the prison, got it to one of the prisoners who shot it at an American soldier who took it safely "in the vest", and then the Americans returned fire. A bloodbath allegedly ensued. It seems a very unlikely tale, and it is a characteristic of Morris's style that he never challenges his interviewees directly: just lets them talk, and lets us decide.

Some of Morris's tricksiness is intrusive. He sometimes shifts his talking heads to different positions in the frame, perhaps to hint at the slipperiness of differing points of view. The subject of a previous film, Robert McNamara in The Fog of War, was treated far more respectfully: he stayed still. There is syrupy, emotional music from Danny Elfman, and artful tableaux interspersed within the talk. Something closer to the austerity of Claude Lanzmann's Shoah would have been more in order.

This was a double-edged horror. The pictures, so vital for proof, were a part of the abuse itself. The Abu Ghraib scandal was a product of the digital age: ordinary roll-film cameras or Polaroids might have been too conspicuous, there would be no facilities for development, and any resulting prints might have been confiscated or lost. But digital images, immediately accessible and so easily transferable and reproducible, and with ineradicable date and time stamps, were the captors' undoing.

Watching this film is the grimmest experience imaginable; I'm almost tempted to say that it should be subject to the special Restricted-18 certificate from the British Board of Film Classification. Of course, abuses and sadistic photo mementoes are nothing new in war and there have been horrors far greater in scale. But for American military personnel to descend to Saddam's level was one of the worst moments in US history, and Morris's film reveals the truth: the poisonous Abu Ghraib pictures were not merely an American scandal but a human catastrophe.